The invention is directed to a supervisory circuit for improving the reliability of redundant channel control systems. Such control systems are used, for example, in the control of nuclear reactors. The present invention is particularly suited for use in nuclear reactor control systems, but it may be utilized with any other control systems where high reliability and availability are desired.
In general, in the prior art, the reliability of a redundant channel control system can only be improved greatly by using three identical channels in parallel. A two-channel system with a simple gating mechanism between them typically does not have better reliability than a single-channel system, and the cost of additional redundant channels is often prohibitive.
Supervisory circuits for improving the reliability of redundant channel control systems are known in the prior art. The major disadvantage with most of these prior art circuits is that a failure in one of the control channels results in a fluctuation of the overall gain of the system. In numerous applications, this change in gain is not tolerable and the system cannot be used. Thus, often it is preferable that the supervisory cicuit operate out of the main path of current flow with only switches operating between the outputs of the redundant channel control system.
One prior art redundant channel supervisory circuit which operates in this manner is described in U.S. Pat. No. 3,800,164 to Miller. According to Miller, the two channels are connected through a circuit designated as the largest value selector to a summing network which provides a single output. The largest value selector utilizes two absolute magnitude detectors one of which is connected to each of the channel outputs. Two field effect transistor switches are used, one connected to each channel output. The absolute magnitude detectors measure the absolute magnitude of the output of each channel. The absolute magnitudes of the outputs of each channel are subtracted and the sign of the difference obtained is utilized to turn on one or the other of the two switches. The supervisory circuit of the Miller patent operates on the assumption that a channel failure will always result in a signal of increasing absolute amplitude. Thus, the largest value selector passes the signal having the smallest absolute amplitude. An obvious disadvantage of the supervisory circuit revealed by Miller is that it will choose the faulted rather than the unfaulted channel in a case where the faulted channel exhibits a decrease in absolute amplitude.